CBN is the hardest material next to diamond, and a sintered body made of CBN is used for making various cutting tools. Japanese Patent Laying-Open No. 53-77811(1978) discloses an example of such a CBN sintered body suitable for cutting tools.
The sintered body disclosed in the above prior art contains 80 to 40 percent by volume of CBN and has a remainder mainly formed of a carbide, nitride, boride or silicide of a transition metal belonging to any one of groups IVa, Va or VIa of the periodic table or a mixture or solid solution compound thereof, with an addition of Al and/or Si. In the above CBN sintered body, the aforementioned compound forms continuous bonding phases in the structure of the sintered body.
In the aforementioned hard sintered body for tools, a bonding compound is prepared of a carbide, nitride, boride or silicide of a transition metal belonging to any one of groups IVa, Va or VIa of the periodic table or a solid solution compound thereof. Since such a compound has an excellent thermal conductivity and high hardness, the sintered body generally exhibits high performance when the same is applied to a cutting tool.
However, when the sintered body which is disclosed in the above Japanese Patent Laying-Open No. 53-77811(1978) and now on the market, is applied to high-accuracy working of hardened steel, for example, the workpiece deteriorates as its surface becomes rough and the dimensional accuracy during the cutting work depends on the cutting conditions. Further, the life of the cutting tool is relatively short. It is believed that the workpiece surface roughness and its low dimensional accuracy are due to an insufficient strength of the cutting edge or due to progress of wear caused by CBN particles falling out of the tool.